refactor

- Remove standalone sra_krl/ library, inline kernels as C++ template
  specializations under impl/pq_code_distance/
- Replace #ifdef __aarch64__ with COMPILE_SIMD_ARM_NEON and
  SIMDLevel::ARM_NEON templates
- Convert .c files to .cpp, drop LANGUAGE CXX workarounds
- No algorithmic changes, benchmarks consistent with prior results

Author: littleniuer

faiss/CMakeLists.txt

@@ -25,7 +25,11 @@ set(FAISS_SIMD_NEON_SRC
   impl/fast_scan/impl-neon.cpp
   impl/scalar_quantizer/sq-neon.cpp
   impl/approx_topk/neon.cpp
+  impl/pq_code_distance/pq_code_distance-neon.cpp
+  impl/ProductQuantizer-neon.cpp
   utils/simd_impl/distances_aarch64.cpp
+  utils/simd_impl/distances_neon.cpp
+  utils/simd_impl/matrix_transpose_neon.cpp
 )
 set(FAISS_SIMD_SVE_SRC
   impl/pq_code_distance/pq_code_distance-sve.cpp
@@ -431,7 +435,12 @@ if(NOT WIN32)
   endif()
 endif()
 target_sources(faiss_sve PRIVATE ${FAISS_SIMD_NEON_SRC} ${FAISS_SIMD_SVE_SRC})
-target_compile_definitions(faiss_sve PRIVATE COMPILE_SIMD_ARM_NEON COMPILE_SIMD_ARM_SVE)
+target_compile_definitions(faiss_sve PUBLIC COMPILE_SIMD_ARM_NEON COMPILE_SIMD_ARM_SVE)
+# ProductQuantizer-neon.cpp uses vdotq_s32/vdotq_u32 which require +dotprod
+set_source_files_properties(impl/ProductQuantizer-neon.cpp
+  TARGET_DIRECTORY faiss_sve
+  PROPERTIES COMPILE_OPTIONS "-march=armv8.2-a+dotprod"
+)
 
 # =============================================================================
 # Dynamic Dispatch Mode
@@ -462,12 +471,17 @@ if(FAISS_OPT_LEVEL STREQUAL "dd")
       )
     elseif(CMAKE_SYSTEM_PROCESSOR MATCHES "(aarch64|arm64|ARM64)")
       # ARM NEON is always available on aarch64, no special compiler flags needed
-      target_compile_definitions(faiss PRIVATE COMPILE_SIMD_ARM_NEON COMPILE_SIMD_ARM_SVE)
+      target_compile_definitions(faiss PUBLIC COMPILE_SIMD_ARM_NEON COMPILE_SIMD_ARM_SVE)
       # Per-file SVE flags (NEON needs no flags on aarch64)
       set_source_files_properties(${FAISS_SIMD_SVE_SRC}
         TARGET_DIRECTORY faiss
         PROPERTIES COMPILE_OPTIONS "-march=armv8.2-a+sve"
       )
+      # ProductQuantizer-neon.cpp uses vdotq_s32/vdotq_u32 which require +dotprod
+      set_source_files_properties(impl/ProductQuantizer-neon.cpp
+        TARGET_DIRECTORY faiss
+        PROPERTIES COMPILE_OPTIONS "-march=armv8.2-a+dotprod"
+      )
     endif()
   endif()
 endif()
@@ -476,8 +490,15 @@ endif()
 # and NEON source files are always compiled into the main faiss target.
 # (On x86, AVX2/AVX512 are optional and only compiled per opt_level above.)
 if(CMAKE_SYSTEM_PROCESSOR MATCHES "(aarch64|arm64|ARM64)")
-  target_compile_definitions(faiss PRIVATE COMPILE_SIMD_ARM_NEON)
+  target_compile_definitions(faiss PUBLIC COMPILE_SIMD_ARM_NEON)
   target_sources(faiss PRIVATE ${FAISS_SIMD_NEON_SRC})
+  # ProductQuantizer-neon.cpp uses vdotq_s32/vdotq_u32 which require +dotprod
+  if(NOT FAISS_OPT_LEVEL STREQUAL "dd")
+    set_source_files_properties(impl/ProductQuantizer-neon.cpp
+      TARGET_DIRECTORY faiss
+      PROPERTIES COMPILE_OPTIONS "-march=armv8.2-a+dotprod"
+    )
+  endif()
 endif()
 
 if(FAISS_ENABLE_SVS)

faiss/IndexIVFPQ.cpp

@@ -816,6 +816,8 @@ struct IVFPQScannerT : QueryTables {
     }
 
     float dis0;
+    mutable std::vector<float> dis_buffer;
+    mutable std::vector<size_t> idx_buffer;
 
     void init_list(idx_t list_no, float coarse_dis_in, int mode) {
         this->key = list_no;
@@ -931,6 +933,59 @@ struct IVFPQScannerT : QueryTables {
         }
     }
 
+    /// Batch version using pq_code_distance_batch for ARM_NEON optimization
+    template <class SearchResultType>
+    void scan_list_with_table_batch(
+            size_t ncode,
+            const uint8_t* codes,
+            SearchResultType& res) const {
+        if (dis_buffer.size() < ncode) {
+            dis_buffer.resize(ncode);
+        }
+
+        pq_code_distance_batch(
+                pq.M, pq.nbits, ncode, codes, sim_table, dis_buffer.data(), dis0);
+
+        for (size_t j = 0; j < ncode; j++) {
+            if (!res.skip_entry(j)) {
+                res.add(j, dis_buffer[j]);
+            }
+        }
+    }
+
+    /// Batch version with IDSelector pre-filtering for ARM_NEON optimization.
+    template <class SearchResultType>
+    void scan_list_with_table_batch_sel(
+            size_t ncode,
+            const uint8_t* codes,
+            SearchResultType& res) const {
+        // Build index list of passing entries
+        if (idx_buffer.size() < ncode) {
+            idx_buffer.resize(ncode);
+        }
+        size_t npass = 0;
+        for (size_t i = 0; i < ncode; i++) {
+            if (!res.skip_entry(i)) {
+                idx_buffer[npass++] = i;
+            }
+        }
+        if (npass == 0) {
+            return;
+        }
+        if (dis_buffer.size() < npass) {
+            dis_buffer.resize(npass);
+        }
+        // Compute distances only for passing entries
+        for (size_t j = 0; j < npass; j++) {
+            dis_buffer[j] = pq_code_distance_single(
+                    pq.M, pq.nbits, sim_table,
+                    codes + idx_buffer[j] * pq.code_size) + dis0;
+        }
+        for (size_t j = 0; j < npass; j++) {
+            res.add(idx_buffer[j], dis_buffer[j]);
+        }
+    }
+
     /// tables are not precomputed, but pointers are provided to the
     /// relevant X_c|x_r tables
     template <class SearchResultType>
@@ -1239,7 +1294,21 @@ struct IVFPQScanner : IVFPQScannerT<idx_t, METRIC_TYPE, PQCodeDist>,
             assert(precompute_mode == 2);
             this->scan_list_polysemous(ncode, codes, res);
         } else if (precompute_mode == 2) {
-            this->scan_list_with_table(ncode, codes, res);
+            if constexpr (PQCodeDist::simd_level == SIMDLevel::ARM_NEON) {
+                if (this->pq.nbits == 8) {
+                    if constexpr (use_sel) {
+                        // ARM_NEON + IDSelector: pre-filter then batch compute
+                        this->scan_list_with_table_batch_sel(ncode, codes, res);
+                    } else {
+                        // ARM_NEON, no IDSelector: full batch compute
+                        this->scan_list_with_table_batch(ncode, codes, res);
+                    }
+                } else {
+                    this->scan_list_with_table(ncode, codes, res);
+                }
+            } else {
+                this->scan_list_with_table(ncode, codes, res);
+            }
         } else if (precompute_mode == 1) {
             this->scan_list_with_pointer(ncode, codes, res);
         } else if (precompute_mode == 0) {